Characteristic for heterodyne receivers is a mixing stage which mixes the received signal with a reference wave generated by a local oscillator. If the signal received by the mixing stage is an optical signal and the reference wave is an optical wave, the apparatus is an optical heterodyne receiver.
The optical signal input is fed into a mixing stage e.g. a PIN-diode. This mixing stage is connected to an optical oscillator. The mixing stage produces a difference frequency. At the output of the mixing stage an intermediate frequency (IF) is made available. Preferentially, an intermediate frequency filter is provided as an intermediate frequency stage.
Such an optical heterodyne receiver is known from the periodical FREQUENZ 41 (1987) 8, pages 201 thru 208.
As for the frequency tuning of a heterodyne receiver, two possible ways are known: The first is to keep constant the intermediate frequency for which the IF stage is set, while the frequency of the local oscillator is tuneable. For optical heterodyne receivers this has the advantage of wide optical tuning range. Furthermore, the IF filter needs to be tuned to just one fixed intermediate frequency. A disadvantage is that the optical oscillator must be continuously tuneable, which results in stability difficulties. Its linewidth is generally relatively broad, so that phase noise does occur.
Alternatively, it is known to provide a fixed frequency of the optical oscillator. This frequency, thus, can not be varied. Then, however, the intermediate frequency stage is tuneable in order to have the optical receiver tuneable. This has the advantage of a good stability of the optical heterodyne receiver, however the disadvantage of only a small available optical tuning range.